One would like to be able to access the value of the pressure in a cylindrical housing filled with a gaseous mixture, for example in order to measure the internal pressure of a fuel rod of a nuclear power plant reactor.
One would also like to determine the molar mass of the aforementioned gaseous mixture.
In order to access this type of information, in general it is necessary to use destructive methods, such as piercing of the housing.
One can also use methods based on the presence of a radioactive tracer gas (such as Krypton 85, for example). However, these methods are not applicable when one wishes to measure the pressure in a tube which is part of a bundle of tubes containing the same radioactive tracer.
We know from FR 2 739 925 an acoustic sensor comprising:                at least one transducer generating an acoustic wave and/or receiving an acoustic wave in return;        a glass bar to transmit the acoustic waves, and        a liquid coupling layer of the sensor with the rod, the layer having a defined thickness λ/4, λ corresponding to twice the acoustic thickness of the wall of the rod.        
The sensor makes it possible to derive the pressure of the gas in the void volume of the fuel rod, thanks to the amplitude of the waves reflected in the transducer.
The sensor does, however, have drawbacks.
First, it only allows measurement of the pressure of the gas, and not of its molar mass.
Furthermore, the liquid coupling layer in λ/4 allows good transmission of the acoustic waves in the rod, but only in a small frequency interval around the resonance frequency of a stack formed by the sensor and the rod.
Moreover, the amplitude of the resonances of the gas is of course sensitive to the pressure, but also to disturbances or poorly known sizes, such as the absorption of the gas or the flaws of the rod walls. Thus, even after calibration, the precision of the measurements remains low.
Lastly, the measuring method does not make it possible to work with rods containing an object causing dispersion of the waves, such as a spring.
Furthermore, WO 00/73781 discloses a housing characterization technique by a remote sensor (and not in contact, as in FR 2 739 925) working through vibrations of the housing (and not of the gas, as in FR 2 739 925) and furthermore in much smaller frequency fields than those disclosed by FR 2 739 925.